Spin-on filters have been employed in a variety of applications including hydraulic systems and engine lubrication systems. Such filters generally include a filter element within a housing with a cover or nut plate secured at one end of the housing by which the filter can be screwed onto or off of a filter head. A central opening surrounded by a plurality of smaller openings is provided in the cover to direct flow through the filter element contained within the housing of the filter. In an inside/out flow arrangement, pressurized, unfiltered fluid (such as the lubricating oil used in a diesel engine) enters the central opening and exits through the surrounding openings after passing through the filter element within the housing. In an outside/in flow pattern, the pressurized, unfiltered fluid enters the surrounding openings and exits through the central opening after passing through the filter element. A circular gasket is provided on a top surface of the cover to serve as a seal between the filter and the filter head. A spring is provided at the lower end of the housing to push the filter element in sealing engagement with the underside of the nut plate that forms the cover.
Although satisfactory in low and medium applications, generally spin-on filters of the prior art have not been satisfactory for use in high-pressure applications such as in hydraulic transmission pumps, where surges of 1,000 psi or more can occur. Most spin-on filters currently available include covers constructed of a stamped steel disk, and a relatively thinner secondary disk spot welded thereto. The base disk includes an extruded, relatively shallow, internally threaded neck portion by which the filter can be connected to a filter head. Flow openings are punched into the base disk around the neck portion. The lip at the open end of the housing is connected, by means of a rolled lock seam, to the periphery of the secondary disk which is also formed to serve as a seat for the external gasket. In this design, fatigue failure is most likely to occur at the rolled lock seam or at the spot welds. A burst failure is most likely to occur either upon bending of the cover (which allows leakage to pass the external gasket) or upon unfolding of the rolled seam. Thus, prior art spin-on filters have been susceptible to failure at the cover and/or at the connection between the cover and housing. Welding of the housing and cover is often unacceptable due to the incompatibility of housing and cover materials such that a satisfactory weld cannot be formed.
To solve these problems, the applicants developed a novel high-strength, spin-on filter in which a reliable and durable seal between the cover and the open end of the housing was achieved without the use of either a rolled lock seam, or a weld by means of a bead-lock arrangement. This filter is described and claimed in U.S. Pat. No. 5,080,787, assigned to the Fleetguard, Inc., the entire specification of which is incorporated herein by reference. In this particular filter, a round cover formed from die-cast metal is provided which is circumscribed around its outer edge by both an upper, C-shaped groove and a lower groove having a rectangular cross section which seats an O-ring. The die-cast cover further includes a centrally located, threaded aperture which can be screwed onto the nipple of a filter head, and a plurality of oil outlet openings surrounding the threaded, centrally-located aperture. On the top surface of the die-cast cover, a circular groove is provided which circumscribes the outlet openings that surround the threaded aperture. This groove receives a gasket which creates a seal between the top surface of the cover of the filter and the filter head when the threaded aperture of the filter cover is screwed onto the threaded nipple of the filter head. In the method of assembling this prior art filter, the filter element is first placed into the interior of the housing. An O-ring is then seated around the second groove which circumscribes the lower part of the die-cast cover, and the cover is then inserted into the open end of the housing until the C-shaped groove which circumscribes the upper portion of the housing is positioned adjacent to the upper periphery of the housing. A roller is then used to inwardly deform the metal around the periphery of the housing in conformance with the C-shaped groove in the cover in what is known in the art as a "spin beading" operation. The spin-beading secures the cover to the housing without the need for rolled lock seams which, as previously pointed out, are proven to rupture when exposed to high pressures. The use of a spin-beading operation to secure the cover to the housing, instead of a lock seam, allows this filter to be assembled rapidly and inexpensively.
While the filter disclosed and claimed in U.S. Pat. No. 5,080,787 represents a substantial advance in the art, the applicants have observed two areas where improvement would be desirable. First, while the diecast cover used in this filter works well for its intended purpose, it is unfortunately expensive as compared to covers formed from one or more stamped metal plates. Secondly, because the seal between the cover and the housing is dependent upon the proper compression of the O-ring between the cover and the housing, the dimensional tolerances of the O-ring, and the depth of the groove in the cover that seals it are narrow. These narrow tolerances prevent the substitution of a less precisely dimensioned, but lower cost lathe-cut gasket for the O-ring. Thirdly, when the cover is inserted into the open end of the housing incident to the assembly operation, sheer forces are applied to the O-ring that has been previously seated around the lower circumferential groove of the cover which are capable of either damaging the O-ring, or rolling it out of the lower circumferential groove of the cover, thereby destroying the cover seal while the application of a lubricant to the seated O-ring prior to the insertion of the cover into the housing solves much of this problem, it also adds an unwanted step in the assembly operation.
Clearly, it would be desirable if an improved filter could be developed which maintained all of the structural and assembly advantages associated with the filter disclosed and claimed in the '787 patent, but whose cover could be replaced with a lower cost cover assembly formed from stamped metal plates and lathe-cut gaskets. It would further be desirable if the assembly of such an improved filter did not apply unwanted sheer forces onto any of the sealing O-rings or gaskets during the assembly of the filter, and did not require the application of any O-ring or gasket lubricants.